Fiber devices are convenient for generation and delivery of optical beams for numerous applications due to their small size and flexible beam delivery. To increase the available optical power for some applications, optical beams from two or more laser diodes are combined in a single fiber device. For example, some metal cutting applications require output powers of 2-100 kW or more from apertures of 50-100 μm in diameter. Typical fiber lasers are limited in output power to less than about 400-800 W and beams from multiple fiber lasers must be combined to achieve these power levels.
An example optical system for combining such beams is illustrated in FIG. 1. Fiber lasers outputs 101-104 deliver respective optical fluxes to collimating lenses 111-114 that produce collimated beams that are directed by a lens 115 to a common area 118 on an input surface 120 of an optical fiber 122 or other fiber or non-fiber optical device. As shown in FIG. 1, the lenses 111-114 are situated so that each of the optical beams is incident to the input surface 120 at a different angle of incidence. The optical fiber 122 must have a sufficiently large numerical aperture so as to receive the beams over these angles of incidence. Otherwise, the optical power directed into the fiber 122 is unlikely to be delivered to an output 124 of the fiber 122, thus diminishing the effectiveness of coupling the beams to the common area 118.
While the configuration of FIG. 1 can be used, it tends to pose significant mechanical alignment problems that can render beam output powers and beam output power distribution unstable. Typically, fibers used to receive overlapping beams as shown in FIG. 1 are multimode fibers having substantial numerical apertures and support propagation in a large number of optical modes. For such fibers, slight variations in coupling geometry associated with the fiber laser outputs 101-104, the lenses 111-114, the input surface 120, and the fiber 122 can produce variations in the available optical power at the fiber output and substantial variations in the distribution of optical power at the output 124. For applications that require constant, uniform, or otherwise well controlled powers and power distributions, these variations are unacceptable, and improved methods are needed.
While beams propagating in more than one single mode fiber can be combined by arranging the fibers in a bundle, such an arrangement does not provide a suitable combined beam. Optical fibers must have diameters of at least between 60-100 μm for mechanical strength so that a fiber bundle using such fibers would necessarily be larger than 50 μm. While combined beams delivered from such a bundle could be demagnified, demagnification produces unacceptable increases in beam numerical aperture.
Single mode beam combiners are disclosed herein that can produce superior optical powers and power distributions. In some disclosed examples, beam combiners comprise two or more single mode fibers, each having an input surface configured to receive a respective input optical beam. Each fiber has a core and a cladding along which the respective received optical beams propagate. In addition, each fiber includes a tapered portion, wherein the tapered portions of the two or more fibers are situated so as to define a bundled fiber portion having a combiner output surface. The tapered portions have tapers sufficient so that the input beams propagate in a lowest order mode at the fiber input surfaces but are either expanded in size while remaining guided by the cores or become unguided by the cores and guided in the multi-mode inner cladding formed by the fused cores of the input fibers. In the case of light guided by the inner cladding, the outer cladding may be formed by an air interface or alternatively by a lower index transparent material. In some embodiments, the input surfaces of the two or more single mode fibers are situated on flexibly separable fiber portions. In other examples, a tapered combiner cladding is in optical contact with and surrounds the bundled portion, the tapered combiner cladding having a refractive index that is less than a refractive index of the single mode fiber claddings. In other representative examples, the tapered portion of each of the single mode fibers is such that the bundled portion of the fibers and the combiner cladding form a multimode optical waveguide. In still other examples, a guide fiber has an input surface and extends to the combiner output surface, wherein the guide fiber is configured to deliver visible optical radiation to the output surface.
Illuminators comprise a plurality of single mode input optical fibers and a beam combiner having an input coupled to the input optical fibers and an output configured to deliver a combined beam. The beam combiner includes a plurality of tapered fiber portions corresponding to the single mode input optical fibers, wherein the tapered fiber portions are single mode waveguides or few mode waveguides that have waveguide modes corresponding to the input fiber modes at the coupling to the input fibers. Few mode waveguides typically are operated so as to have V-numbers of less than about 5. The tapered fiber portions are tapered so that the single waveguide modes expand to have mode field diameters that are at least twice the mode field diameter of the input optical fibers. An outer cladding may be situated about the tapered fiber portions so as to define a multimode waveguide in combination with the tapered fiber portions. Alternately, a waveguide may be formed by the air glass interface between the fused fibers and the surrounding air. In some examples, lasers such as diode lasers or fiber lasers are coupled to the single mode input fibers. In other examples, at least one of the tapered fiber portions is rare earth doped and is configured to receive pump radiation from other tapered fiber portions. In other embodiments, the single mode input fibers have mode field diameters of between about 5 μm and 15 μm, and the tapered fiber portions are tapered by at least a factor of two, five, or ten from an input to an output. In additional representative examples, an output multimode fiber is optically coupled to the tapered fiber portions. In some case the output from the tapered device may be coupled to pump a fiber laser. For example the output from an array of 1020 nm Yb-doped double clad fiber lasers may be combined to pump a Yb laser.
Optical amplifiers comprise a plurality of tapered pump fibers fused to a doped signal fiber and situated in an outer cladding. The pump fibers are single mode fibers tapered and fused so as to define a multimode core, and the signal fiber is a rare earth doped fiber operable to provide optical gain in response to pump radiation from the tapered pump fibers. In other examples, the multimode core defines a multimode fiber in combination with the outer cladding.
Tapered couplers comprise a plurality of tapered, fused single mode fibers enclosed in a glass ferrule or twisted around each other prior to fusing, the fibers tapered by a factor of at least 2. In some examples, at least one of the tapered single mode fibers includes at least a first core and a second core, and the second core defines a confined waveguide mode at a smaller end of the taper. In some embodiments, at least one of the tapered single mode fibers includes a core and a cladding, and at a smaller end of the taper, a waveguide mode extends to at least substantially all of a cladding cross-sectional area. In typical examples, the fibers are tapered by between a factor of 5 and 10.
Methods comprise inserting a plurality of single mode optical fibers through a glass ferrule and heating the fibers and the ferrule. The heated fibers and ferrule are stretched so as to fuse and taper the fibers and the ferrule to form a tapered bundle having an output surface. The single mode fibers are tapered so that that a multimode fiber is defined by the fused fibers and the ferrule or glass air interface. In other examples, at least one of the single mode fibers is a fiber having a first core and second core, and the second core defines a single mode waveguide after tapering. In further examples, the single mode fibers are tapered so that a mode defined by a fiber core expands to fill at least a tapered single mode fiber cladding.
Beam combiners comprise two or more few mode optical fibers having respective tapered portions that are situated so as to define a tapered fiber bundle. The tapered portions have tapers sufficient so that a mode field diameter of a lowest order mode associated with the few mode optical fibers is expanded by a factor of at least two. In some examples, the lowest order mode field diameter is expanded in the tapered portions to be substantially the same as a cladding diameter of the few mode optical fibers. In other examples, the tapered portions are configured so that a lowest order mode of each of the few mode optical fibers is substantially unguided by respective few mode optical fiber cores. In still other examples, the tapered portions are configured so that a lowest order mode of the few mode optical fibers is expanded so as to substantially occupy a cross-sectional area of the tapered fiber bundle. According to some examples, the few mode optical fibers have V-numbers of less than 5 or are single mode optical fibers. In some examples, input surfaces of the two or more few mode fibers are situated on flexibly separable fiber portions. In additional embodiments, a tapered combiner cladding is in optical contact with and surrounds the bundled portion, the tapered combiner cladding having a refractive index that is less than a refractive index of the few mode fiber claddings. In some examples, the tapered portions of each of the few mode fibers are fused to form the bundled portion so that the fused fibers and the combiner cladding form a multimode optical waveguide. According to some representative examples, the tapered portions are configured so that a lowest order mode of the few mode optical fibers is expanded so as to substantially fill a cross-section of the fused bundled portion.
In some examples, a tapered fiber bundle has an output surface, and a beam combiner further comprises a guide fiber extending through the bundled portion to the output surface. The guide fiber can be selected to permit the delivery of visible light to a work surface. In some examples, the beam combiner includes a single mode signal fiber having a corresponding tapered portion in the tapered fiber bundle, wherein the signal fiber is configured to be a few mode optical waveguide or a single mode optical waveguide in the tapered portion. In other embodiments, the beam combiner includes a double core fiber having a first core and a second core and having a tapered portion that is situated in the tapered fiber bundle, wherein the tapered second core defines a few mode fiber. Typically, the double core fiber is a most central fiber in the tapered fiber bundle. In some examples, the few mode fibers are double core fibers having a first core and a second core, wherein a most tapered portion of the tapered second core defines a few mode optical fiber. In additional examples, the beam combiner includes a signal fiber having a corresponding tapered portion in the tapered fiber bundle, wherein the signal fiber is configured to be a few mode optical fiber in the tapered portion.
Fiber assemblies comprise a beam combiner having a plurality of few mode input optical fibers that are fused and tapered to form a fiber bundle having an output surface. The few mode fibers are tapered so that optical beams received by the few mode input optical fibers are expanded by the tapering. An output fiber is optically coupled to the output surface of the beam combiner. In some examples, the output fiber is a double clad fiber having a core and an inner cladding, wherein the beam combiner is configured to optically couple the few mode input optical fibers and the inner cladding. In further examples, the beam combiner includes a signal fiber that is tapered in the fiber bundle so as to define a few mode fiber at the output surface, and is optically coupled to the core of the double clad fiber. In additional examples, a mode field diameter of a lowest order mode of the tapered signal fiber is substantially the same as a mode field diameter associated with the double clad fiber core. In other examples, the signal fiber is a double core fiber that is tapered in the fiber bundle so that a lowest order mode associated with a first core of the double core fiber expands so as to substantially fill a second core of the double core fiber. In still other examples, the few mode input optical fibers have claddings having refractive indices that are less than a refractive index of the outer cladding of the double core signal fiber so that the few mode optical fibers are optically coupled to the outer cladding. In some example, radiation sources are coupled to corresponding few mode input optical fibers. In some examples, the few mode fibers are single mode fibers.
Illuminators comprise a plurality of single mode input optical fibers and a beam combiner having an input coupled to the input optical fibers and an output configured to deliver a combined beam. The beam combiner includes a plurality of tapered fiber portions corresponding to the single mode input optical fibers, wherein the tapered fiber portions are single mode waveguides that have waveguide modes corresponding to the input fiber modes at the coupling to the input fibers. The tapered fiber portions are tapered so that the single waveguide modes expand to have mode field diameters that are at least twice the mode field diameter of the input optical fibers. In some examples, illuminators include an outer cladding situated about the tapered fiber portions so as to define a multimode waveguide in combination with the tapered fiber portions. In typical examples, laser diodes, fiber lasers, or other lasers are coupled to the single mode input fibers. In some examples, the single mode input fibers have mode field diameters of between about 5 μm and 15 μm, and the tapered fiber portions are tapered by at least a factor of between two and five from an input to an output. In other examples, an output multimode fiber is optically coupled to the tapered fiber portions.
Optical amplifiers comprise a plurality of tapered pump fibers fused to a single mode signal fiber to form a fiber bundle, wherein the pump fibers are single mode fibers tapered and fused so as to define a multimode core. A rare earth doped fiber is optically coupled to the fiber bundle, wherein the rare earth doped fiber is operable to provide optical gain in response to pump radiation from the tapered pump fibers. In some examples, an outer cladding has a refractive index that is less than a refractive index of pump fiber claddings.
Tapered couplers comprise a plurality of tapered, fused single mode fibers enclosed in a glass ferrule, the fibers tapered by a factor of at least 2. In some examples, at least one of the tapered single mode fibers includes at least a first core and a second core, and the second core defines a confined waveguide mode at a smaller end of the taper. In additional examples, at least one of the tapered single mode fibers includes a core and a cladding, and at a smaller end of the taper, a waveguide mode of the at least one tapered single mode fibers extends to at least substantially all of a cladding cross-section. In typical examples, the fibers are tapered by between a factor of 5 and 10.
The foregoing and other objects, features, and advantages of the technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.